Added "srgba8" and "sgray8" sRGB color types, along with associated pixel formats.
Implemented linear-light rendering in examples and Win32 platform support (other platforms require corresponding changes).
Fixed a few bugs.

git-svn-id: https://agg.svn.sourceforge.net/svnroot/agg@63 ab9de31f-a152-4fe9-8809-95f286f9ca89